if you kill the mouse and feed it to the snakes it then becomes pre- killed closer to f/t but not there yet. humanely killing it would consist of a c02 chamber ect.. ect.. the quickets way to uthanise without further expences it is to stick it in a pouch( bag, sock ect.) and hit it over a table or the floor, make sure it is a solid enough hit to brake its neck or it will suffer, also make sure its a solid pouch or you might fling the mouse across the room if the bag brakes mid strike.

now the key to feeding f/t is to try.
as long as the rat/mouse is thawed out completely, dry and warm enough to pass as pray the snake should eat it, maybe not the 1st offering or the second but it will.. i would make sure the f/t ( frozen thawed) is thawed out properly, dry and warm enough to pass as pray, then i would sent it with what i was feeding the snake with before.
to sent , simply rub the f/t on the soiled mouse bedding or on the animal itself so the snake perceives it to be what it was eating before.
after a few successful f/t scented feedings i would try a unscented f/t.

hope that helps, good luck!!